Nineteen firefighters have been killed battling a fast-moving wildfire in Arizona, in one of the deadliest incidents of its kind in the United States in 80 years.

The elite firefighters died while racing to contain a wildfire at Yarnell Hill, north of Phoenix.

The deadly blaze came amid baking temperatures and tinder-dry conditions across the US south-west, with records broken over the weekend in Arizona and California,

US president Barack Obama, who is travelling in Africa, issued a statement paying tribute to those who lost their lives in what he called a "terrible tragedy".

"They were heroes - highly-skilled professionals who, like so many across our country do every day, selflessly put themselves in harm's way to protect the lives and property of fellow citizens they would never meet," the statement said.

The Yarnell Hill fire, which is believed to have been ignited by lightning, broke out on Friday (local time).

Local forestry officials say the blaze has already covered over 800 hectares and has spread rapidly through the dry, forested area amid strong winds.

"They evacuated the town of Yarnell this past afternoon, that was 400 residents, 50 houses nearby in the Peeples Valley.

"All of these people are being sent to a nearby community college where Red Cross has sent up a care centre."

Worst fire-fighting incident since 1933

Figures from the National Fire Protection Association show that the Arizona deaths are the worst firefighter fatalities from a wildfire since 29 firefighters died fighting a blaze in Los Angeles' Griffith Park in 1933.

Sunday's deaths also mark the largest loss of firefighter lives on US soil since the September 11, 2001 attacks on the World Trade Centre, when 340 firefighters were killed.

A local news website, the Arizona Republic, reported that federal help was set to arrive on Monday.

As tributes poured in for the fallen firefighters, Arizona's governor Jan Brewer said it was "as dark a day" as he could remember.

"It may be days or longer before an investigation reveals how this tragedy occurred, but the essence we already know in our hearts: fighting fires is dangerous work," she said.

"The risk is well-known to the brave men and women who don their gear and do battle against forest and flame."

One Arizona state forestry official told CNN the firefighters had been digging a fire line to contain the blaze.

"In normal circumstances, when you're digging fire line, you make sure you have a good escape route, and you have a safety zone set up," Art Morrison said.

"Evidently, their safety zone wasn't big enough, and the fire just overtook them."

Arizona senator Jeff Blake said on Twitter that he was "sick with the news".

19 firefighters dead battling #yarnellfire. I am sick with the news. Prayers go out to all affected.

The Yarnell Hill wildfire is the worst of several blazes raging across the dry state and comes two weeks after a blaze in Colorado killed two people and burned down 360 homes, the western state's most destructive blaze ever.